1. Field of the Invention
The present invention relates to an image processing apparatus and an image processing program product. More specifically, the present invention relates to an image processing apparatus and an image processing program product for processing image data obtained by scanning of a document image having a ground pattern for preventing replication.
2. Description of the Related Art
Since performance of an image processing apparatus such as a copier or a printer has been improved, a high quality replica of a document image can be easily made utilizing such apparatus. Therefore, it becomes critical to prevent improper replication.
To prevent improper replication of a document image, Japanese Laid-Open Patent Publication No. 2002-237940, for example, suggests an image processing apparatus which converts a classified document into a color which is hard to reproduce by copying in a step of superposing a document image on a background image and outputting the result. In addition, Japanese Laid-Open Patent Publication No. 2004-274092 suggests a technique to include in a document image a ground pattern for preventing replication.
FIGS. 36A and 36B show an example of a ground pattern contemplated in the present invention. As shown in FIG. 36B, which is an enlarged view of the ground pattern shown in FIG. 36A, the ground pattern is formed with a combination of a plurality of patterns having different resolutions. More specifically, referring to FIG. 36B, the ground pattern is a background image of a combination of a pattern 1 (a “ground” portion) formed with lines of a high screen ruling and a pattern 2 (a “pattern” portion) formed with dots of a low screen ruling.
FIGS. 37A and 37B show a specific example of a document image having a ground pattern superposed as a background image. When the document image having the ground pattern shown in FIG. 37A is copied, pattern 1 which is a pattern of thin lines is obtained as pale gray dots, while pattern 2 which is a pattern of large dots is obtained as separate black points with a resolution of a scanner. Thereafter, a pale pattern of pattern 1 is removed with a ground-removal function of a copier and only black points of pattern 2 remain as shown in FIG. 37B, resulting in appearance of a latent image formed with pattern 2 indicating that a resulting document is a copy.
Since an effect of the ground pattern for preventing replication varies depending on a resolution property or an amount of the ground removal of the copier, however, sometimes a sufficient effect cannot be obtained when the ground pattern for preventing replication is scanned with a scanner and converted into electronic data, depending on a setting of a scan. In addition, since the electronic data obtained with the scan can be processed to freely change lightness or the like, the ground pattern for preventing replication can be easily removed by image processing.
FIG. 38 is a flow chart of a specific example of processing executed in a conventional image processing apparatus to erase the ground pattern. FIGS. 39A and 39B show specific examples of images obtained in respective steps of the flow chart of FIG. 38 when the document image having the ground pattern shown in FIG. 37A is specifically processed.
Referring to FIG. 38, by scanning the document image having the ground pattern with a high resolution and inputting electronic data (step S1), the ground pattern is accurately input at a halftone level.
Then, a smoothing filter is applied to this image data to perform smoothing so as to decrease a density of a ground region to which the ground pattern belongs (step S2). FIG. 39A shows a specific example of an image obtained with the smoothing in step S2 when the document image having the ground pattern shown in FIG. 37A is input in step S1. Referring to FIG. 39A, by applying the smoothing filter having a size larger than a width of a line forming pattern 1 and smaller than a size of a dot forming pattern 2 in step S2, a tone difference is generated between pattern 1 and pattern 2, and pattern 2 of at least a prescribed density is deleted by decreasing the density.
Furthermore, a tone is changed so as to remove the ground region to remove the ground pattern (step S3). FIG. 39B shows a specific example of an image obtained with changing the tone in step S3 when the image shown in FIG. 39A is obtained in step S2. Referring to FIG. 39B, the ground pattern for preventing replication can be erased in step S3 by emphasizing a tone difference between the ground pattern, in which pattern 2 is deleted in step S2 and only pattern 1 remains, and the other elements to delete a tone level of the ground pattern.
To prevent such removal of the ground pattern, a technique has been proposed in which a watermark is printed with the ground pattern and a scan itself is prohibited when the watermark is sensed. With this technique, a scan can be prevented independently of a property of a scanner.
An effect as described above, however, cannot be obtained with a document image with a ground pattern which is output from a machine not carrying such watermark technique or an old machine manufactured before introduction of the watermark technique. In addition, distribution of the document also becomes impossible because the scan itself is prohibited.